The objectives of this project are the determination of the nature and biological properties of rickettsial antigens and constituents and the development of procedures for classification of spotted fever group rickettsiae. SDS-PAGE analysis of four strains of rickettsiae representing three serotypes of the spotted fever group differed principally in the position of a major protein with an apparent molecular weight of about 17K. A sedimentable fraction of Sarkosyl-extracted rickettsiae contained predominantly a 17K and a 29K protein. Work is in progress to determine whether the 17K proteins are responsible for type specificity. A number of hybridomas producing antibodies which react with Rickettsia rickettsii in the indirect immunofluorescence test have been produced. One of these monoclonal antibodies protects mice against lethal challenge with R. rickettsii. The other reacts with a common epitope on multiple components of R. rickettsii; possibly this epitope is found on various classes of lipopolysaccharide molecules differing in the length of the polysaccharide side chains.